Serioulsy I doubt you have anything legit.
1. Her Early Game is so Strong!
Urgot. He does more damage and he does it much safer. He is better at farming and plays a bigger role late game.
Nidalee. She does more damage once she hits 6, she is better at farming and far more sustained. She has a bigger impact on her team.
Zilean. He does more damage, and is a great support for any team.
I don't see how Vayne's early game, or total kit is any better than these champions.
2. % Health True Damage is Game Breaking.
Every 3 hits she does 70 + 8% HP. Meaning she does 2.6% + 23 true damage per hit. Less than Irelia until her enemy has 2800HP. She needs to land 3 hits as well.
3. It's just her whole package, she does everything too well!
She has high single-target damage with no escape mechanisms or inherit survivability minus 12 seconds of invisibility when she rolls. One single-target long CD situational stun. No AoE. No Toughness. Low Range.
She is pretty sub-par in teamfights. She can be focused extremely fast and falls to CC very readily. I'd say Nocturne and Poppy are much better Assassins. Nocturne has his semi-global dash and spell shield while poppy has invincibility.
I personally find Vayne boring to play and a headache to be in teamfights, she isn't terribly hard to lane against either. You just have to play aggressively. She can dish out damage but she can't take it.
The closest I ever got to being outlaned by Vayne was as Kog'maw. She killed me once, yet Kog'maw is 10 times better late game. Even a fed Vayne isn't all that scary.
Judging by a recent poll most of the community should agree with this.
Really that is her only problem. People just dont know how to lane against her yet, which is why they think her burst is op.
Anyways OP, your thread sounds very much like a "Please dont nerf x thread, I like it" Her bullets will most definitely be adjusted. She might get some love else where to compensate.